RE: RK3399 DisplaPort no 4K resolution

I too try to get a DisplayPort 4K monitor (AOC U2879VF) working, however no success yet.
I have modified the DTS-file accordingly, and I see a couple of things:

1) 'dumpsys SurfaceFlinger' shows 2 displays: "Built-in screen" and "HDMI screen". The latter is the the DisplayPort 4K monitor. The naming is hardcoded in the Android source, and reading the EDID shows it is the correct monitor (and I have no display connected via HDMI). Only one monitor via HDMI shows only "Built-in screen". I don't think this is an issue, as Android tries to clone to each video output.

2) I guess the VOP_LITTLE seems to be selected for DisplayPort output (and not the VOP_BIG), as the current mode is 2560x1440@60p and the modes list does not contain above this (also not 3840x2160p60), while the monitor is capable.

Resulting in some questions:
1) How can I verify which VOP is "used" for which display?
2) How to make sure VOP_BIG is "used"?
3) Do you have any information about this "dp_status 18a"? Maybe to suspect the monitor ...

additional info: I see a difference between boards, as I have boards with different production dates/stickers
- one board with production date 2017-08-22: see above observations
- one board with production date 2018-03-29: works out of the box! (although only 2560x1440@60p, and not yet 3840x2160@60p)
I don't know whether there is a hardware change, or it is subject to hardware component variations, ...